An osteosynthesis device comprises a fixing plate, at least two pins designed to be screwed into two bone parts, and lock rings for anchoring the pins and thus fastening the fixing plate to the two bone parts.
A major problem in this technical field is to devise an osteosynthesis device offering both efficient locking of the pins, to prevent them coming unscrewed when subjected to mechanical stresses or vibrations, and easy unscrewing of the pin when the practitioner wishes to remove the osteosynthesis device.
EP 1 583 478 discloses an osteosynthesis device in which the axis of the pin and that of the lock ring are at an angle relative to each other. In that osteosynthesis device the holes of the fixing plate are threaded and receive an intermediate ring containing a threaded bore to receive a pin and a lock ring.
In order to fit the osteosynthesis device, the practitioner first drills the fixing plate at least two points selected to enable it to be anchored to the bone part, and then drills this bone part with a drill bit guided perpendicular to the fixing plate. Next, he fits the intermediate ring and screws the threaded pin into the bone part to the desired length. He then places the lock ring onto the pin and screws this lock ring into the intermediate ring.
It can be seen that, because of the angular difference between the axis of the pin and that of the lock ring, the lock ring becomes increasingly wedged into the intermediate ring as it is screwed down. This provides an irreversible locking of the pin relative to the fixing plate.
That device is effective but has a number of drawbacks.
In the first place, there is little tolerance in the angular interval between the pin axis and the lock ring axis. Accurate manufacture of the intermediate ring is therefore required, resulting in a high cost of manufacture.
In addition, although this angular interval results in effective locking, it has the disadvantage that this locking action begins as soon as the lock ring begins to be screwed down. The lock ring can therefore sometimes fail to penetrate sufficiently into the fixing plate.
Lastly, in the case of a thin plate (less than 3 mm, for example), the intermediate ring and the lock ring must be inserted partially into the bone part if the device is to be properly anchored. This means that this prior art device cannot be used on the upper bones (arm, forearm, hands, wrist, face), which are too thin.